Update 4 is now on the PTS, and is due to go live sometime in September. The patch notes (as they stand at the moment, subject to change when it's been tested on the PTS) are available:
http://forums.elderscrollsonline.com/discussion/127711/pts-patch-notes-v1-4-0
Highlights:-
A new group veteran zone with new trials (high level players who prefer to solo, don't panic, there is a solo veteran zone planned, probably coming in update 5 or 6)
Arena fights in the Dragonstar Arena (for groups of 4 players, both normal and veteran modes)
Changes to enchanting so it's not so much of a bloody nightmare to level the skill, and new enchantments that can alter your abilities
A new researchable armour trait, Nirnhoned, which gives a bonus to spell resistance, and an extra rank of the perk that reduces research times
Improvements to all delves in the final zone of each faction (part of a general rollout to improve the size, uniqueness, and boss encounters of delves throughout the game, Cyrodil was done in update 3 and more will be coming in the future)
Grouping improvements including shared quest stages when grouped for a whole number of quests
PFX Prioritization - effects on you and your group will take priority over background stuff, this should provide a big improvement in terms of less combat lag and quicker response, and less delay in seeing combat cues on foes
Also more hireling messages have been added, may seem like a small thing but I'm excited about it!